The present invention relates to an ignition system for an internal-combustion engine, in particular for use in a chain saw or the like, comprising a magnetically conducting core having at least three legs and supporting a charging winding and a triggering winding, a flywheel having at least one magnetic field generating member adapted to cooperate with the magnetically conducting core to cause, when passing, voltages to be induced in the windings, and an electronic switch means adapted to trigger, in response to a trigger pulse generated by the triggering winding, the discharge of a first capacitor, which has been charged by a voltage generated in the charging winding, via the primary of an ignition coil the secondary of which comprises a spark plug.
Internal combustion engines for chain saws and the like often operate at very high speeds, for example up to 14,000 rpm. This means that the ignition system of the engine has to be designed so as to have a large ignition advance in order for the engine to operate properly. At the same time, the ignition advance must not be too large when the engine operates at idling speed or is to be started-up. If the ignition advance is too large at start it can easily happen that reverse motion takes place which may cause injury to the person handling, for instance, a chain saw. At start the person holds the chain saw in a certain grip and pulls a starting strap in order to rotate the engine and have it start. If during the compression phase of the piston an ignition spark should appear early, that is far before the top dead center, the piston may turn causing the engine to change its direction of rotation and pulling the strap back into the machine. This course is very abrupt so that the starting handle can be pulled out of hand of the person handling the saw with inconvenience or injury as a consequence.
In DE-A-38 33 620 an arrangement is described in which the problem is solved which relates to a too large ignition advance and the associated risk of backstroke to take place. A magnetically conducting core in the ignition system is provided with three legs of which, seen in the direction of rotation of the flywheel, a first leg has a first triggering winding while the center leg has a second triggering winding. The third leg, finally, supports the charging winding which generates energy for a capacitor which, by discharge, generates an ignition spark at the secondary of an ignition coil. At start, only the triggering winding provided on the center leg is active causing the ignition advance value to be moderate so that backstroke is avoided. In operation, a contact is closed connecting the triggering winding on the first leg causing the ignition advance, in a single step, to be increased to a suitable operational value. Even if in this way the problem has been solved which relates to the risk of a backstroke taking place, the possibility of additionally increasing the ignition adsvance under operating conditions is limited with the arrangement referred to. In practice, with the arrangement described it is not possible to reach an ignition advance value beyond about 10 degrees whereas about 25 degrees should be required at the high speeds concerned.